1. Field of the Invention
The present invention relates to a rack and pinion type motor-driven power steering apparatus.
2. Description of the Related Art
In a rack and pinion type motor-driven power steering apparatus which assists steering operation by transmitting drive force of a motor to a steered wheel from a pinion shaft via a laterally moving rack, the laterally moving rack which passes through a gear box in which a rack and pinion mechanism is received, is brought into contact with a gear box side in a stopper portion of the rack at a stroke end in order to stop.
If the steering wheel is turned sharply, the stopper portion of the rack comes into banging contact with the gear box side at the stroke end. This results in problems such as the generation of an uncomfortable noise and damage to the gear box is damaged due to an impact load, and excessive motor drive current flow.
Accordingly, there has been proposed an example in which the motor drive current is reduced little by little when the rack reaches the position near the stroke end (refer to Japanese Utility Model Publication No. 8-8943 (patent document 1)). In the patent document 1, a rack end proximity sensor is provided which actuates a time constant circuit during a detecting time when the rack moves close to the stroke end. A current decreasing control circuit damps the motor drive current little by little so as to thereafter control and keep the motor drive current at a comparatively low level current value.
However, in the patent document 1, since the motor drive current is always damped at a fixed damping factor set by a time constant circuit 28 without reference to moving speed of the rack when the rack moves close to the stroke end, if the damping factor is set comparatively large so as not to cause trouble when the steering wheel is sharply turned and the moving speed of the rack is fast, the motor drive current is also damped at the same damping factor near the stroke end so as to be kept at a low level current value, even when the steering wheel is gently turned and the moving speed of the rack is slow. Accordingly, the time for steering in a heavy steering state has to be elongated, and the steering characteristic is deteriorated.